U.S. Pat. No. 5,835,875, filed by the present inventors, which is owned by the assignee of the present invention, discloses a powertrain with planetary gearing, including clutches and brakes that are controlled by a control system with features that are common to the present invention. The transmission of the '875 patent comprises a first compound planetary gear unit with three forward-driving ratios, commonly referred to as a Simpson gearset, and a second gear unit at the torque input side of the first gear unit. Separate pressure-operated clutches and brakes effect upshifting and downshifting. A friction brake for the reaction element of the second gear unit establishes a torque reaction, and an overrunning coupling establishes a torque flow path between two elements of the second gear unit as the friction brake for the second gear unit is released.
Ratio changes in the gearing of the '875 patent are controlled by an electronic microprocessor controller which develops control signals in response to changes in operating variables of the powertrain to actuate four shift solenoids, which in turn control shift valves for actuating and releasing the pressure-operated brakes and clutches.
On a ratio change from the second ratio to the third ratio, it is necessary to exhaust pressure from the apply side of the reaction brake for the second gear unit and from the release side of the reaction brake for the first gear unit. Thus, the brake for the first gear unit becomes applied, and the brake for the second gear unit becomes released in a so-called "swap shift" sequence.
The pressure exhaust flow path for the reaction brake for the first gear unit is combined with a circuit for the clutch for the first gear unit, which is engaged during fourth ratio operation. As the release side of the brake for the first gear unit is depressurized, the apply side of the brake is pressurized. This creates a condition in which a pressure buildup may occur in the clutch because of the common exhaust port for the clutch and the brake located in the clutch circuit. Further, it is necessary in a control system such as that disclosed in the '875 patent to increase the rate of response of the clutch for the first gear unit to commands for clutch engagement and clutch release as ratio changes occur between the fourth ratio and the second ratio or between second ratio and fourth ratio.